For particular applications, for example observation of the earth from satellites, it is desirable to produce linear sensors of great length, more than that which can be produced on a single silicon chip, and for this purpose a plurality of chips may be aligned end-to-end in the direction of their length.
FIG. 1 represents a top view of a row, on a printed circuit board 10, of a plurality of packages B1, B2, B3 . . . each carrying a respective semiconductor chip P1, P2, P3 . . . in the form of an array, each chip being electrically connected to its package by wires welded between connection terminals of the chip and connection terminals of the package.
The drawback of this layout is the presence of blind zones: even if the photosensitive points practically reach the edge of the chip and even if the chip practically reaches the edge of the package, the junction zones at the boundary between two chips will not contain photosensitive points (or pixels in image detection). In fact, the pixels will scarcely reach closer than about fifty micrometers from the edge of the chip, and the chip can only reach the edge of the package with a tolerance of the same order. For pixels having a pitch of about 10 micrometers, about ten pixels risk being absent in each abutment zone of adjacent packages. This is not acceptable because the overall image reconstructed in the course of the relative displacement of the sensor will comprise black columns in the translation direction.
FIG. 2 represents a lateral section of a solution which has already been attempted in order to resolve this drawback: the photographic optics project the image to be detected not directly onto the photosensitive sensor but onto the plane entry face of a set of optical fiber bundles F1, F2, F3 . . . ; the fiber bundles F1, F2, F3 are juxtaposed and leave no gap between them on the side of their entry face; each bundle is deformed by tightening the side turned toward the chip, however, so that its exit face occupies only the useful length of the linear array. The blind zones are thus eliminated. This solution is very expensive, in particular because of the extreme precision which is required in the juxtaposition of the fiber bundles and in the deformation of these bundles so that their exit occupies exactly the active photosensitive length of each array.
FIG. 3 represents a more practical solution which may be envisaged: the chips abut, although in a staggered fashion; they are therefore not aligned, or more exactly they are divided into two groups of aligned chips and the two groups form two parallel rows, each chip of the second group being placed between two separated chips of the first group but offset out of the row of chips of the first group. The distance between two chips of a row is less than the length of the package of a chip. The packages therefore bear against one another at the end, along an edge parallel to the row direction; the ends of the adjacent chips comprise detection zone portions in mutual overlap so that there is absolutely no dead zone. The separation of the row axes of the two groups is equal to the width L of the package. It is perfectly known and, when photographing in translation, taking into account the translation speed V, an image line is reconstructed by combining the information provided by two adjacent chips not at the same time but with a time interval L/V, which compensates for the fact that the chips do not see the same image line at the same time.
This solution suffers from drawbacks associated with the need to reconstruct the final image, with risks of distortion and instability which are commensurately more problematic as the distance between the two chip rows is greater. The distance is moreover equal to the width of the package, that is to say several centimeters.
One possible solution for limiting this drawback is represented in FIG. 4: the chips are all placed at the edge of the package, all the wires connecting the chip to the package being relocated to one side of the chip. The chips are still staggered, and the signal which they provide therefore has to be processed in order to carry out reconstruction, taking into account the offset of the row axis of the chips, although this offset is now practically only the width of the chip and no longer the width of the package.
Unfortunately, this solution is expensive because it requires a particular chip design, having connection terminals on only one side, and it may furthermore require the use of two different components depending on whether the chip belongs to one row or the other because the chips are alternately reversed; reversal, however, is not necessarily compatible with the operation of the chip. This is the case in particular for multilinear arrays operating in TDI (Time Delay Integration) mode, or multispectral imaging arrays having a plurality of lines of pixels corresponding to different colors or ranges of the spectrum. This is because reversal of the chip entails signal processing problems in both cases. This solution is therefore difficult to implement.